This application claims the benefit of Korean Application No. 2001-84398, filed Dec. 24, 2001 in the Korean Industrial Property Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to microwave ovens, and more particularly, to a microwave oven which blocks high frequency noise flowing through ground and power lines.
2. Description of the Related Art
Generally, a microwave oven carries out a cooking operation by converting commercial alternating current (AC) power into a high voltage to drive a magnetron. Since the microwave oven uses the commercial AC power, it is difficult to install and use the microwave oven in transportation vehicles such as cars.
Recently, a microwave oven has been developed to use direct current (DC) power, such as a battery, to cook foods. This type of microwave oven employs an inverter circuit to convert a DC voltage into an AC voltage required to drive a magnetron.
FIG. 1 shows a conventional microwave oven using DC power comprising a DC power source 1, a low voltage unit including a switching unit 2 and an inverter unit 3 connected to a primary side of a high voltage transformer 4, a high voltage unit including a magnetron driving unit 5 connected to a secondary side of the high voltage transformer 4, a magnetron filter box 6, and a controller 10. The magnetron filter box 6 includes a magnetron 7, and a low pass filter which comprises choke coils 8 and feed-through condensers 9.
The switching unit 2 includes a door switch, a low voltage transformer, and a typical noise filter, and supplies operating power, which is voltage-dropped by the low voltage transformer, through a power line connected to the controller 10. The inverter unit 3 comprises a push-pull circuit having switching devices, such as Field Effect Transistors (FETs), and is switched by a control signal from the controller 10 to convert DC power into AC power.
The high voltage transformer 4 induces the AC power converted according to a turn ratio of a primary coil L1 to a secondary coil L2 as a high AC voltage, and supplies the high AC voltage to the magnetron driving unit 5. The magnetron driving unit 5 supplies a high DC voltage of 4000V to the magnetron 7 using a half-wave voltage doubler circuit (not shown) having a high voltage diode and a high voltage condenser.
One end of each of the switching unit 2 of the low voltage unit and the magnetron driving unit 5 of the high voltage unit is grounded through a ground line.
In the conventional microwave oven using the DC power as a power source, since the controller 10 drives the inverter unit 3 at a high usable frequency of 20 to 50 KHz, electrical energy flows are exchanged between the low and high voltage units, thus causing high frequency noise to be widely generated. Furthermore, the high frequency noise flows through the ground lines and radiates to the outside. Moreover, the high frequency noise flows through a power line connected to the controller 10, thereby causing the high frequency noise to disturb a control operation of the controller 10.
Accordingly, it is an object of the present invention to provide a microwave oven which blocks high frequency noise flowing through ground and power lines.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
To achieve the above and other objects of the present invention, there is provided a microwave oven comprising a low voltage unit which converts direct current (DC) power into alternating current (AC) power and supplies operating power, a controller which receives the operating power from the low voltage unit, a high voltage transformer having primary and secondary coils, wherein the high voltage transformer induces the AC power converted by the low voltage unit as a high AC voltage according to a turn ratio of the primary coil to the secondary coil, a high voltage unit which generates a high DC voltage from the high AC voltage induced by the high voltage transformer and supplies the high DC voltage to a magnetron, and a filter unit which blocks high frequency noise while passing a signal of a DC voltage through ground lines to which the low and high voltage units are grounded and a power line connected between the low voltage unit and the controller.
According to an aspect of the present invention, the filter unit includes at least one choke coil which increases impedances of the ground and the power lines.